| Strong resonance structures of photoabsorption spectra of atoms in external fields have attracted much interest in recent years, since they involve many basic questions such as high Rydberg atoms' structure, the matter diamagnetism in extreme conditions and electron correlation, etc. Semiclassical closed-orbit theory has been extensively used to explain the photo-absorption spectra of atoms in external fields due to its clear physical picture and wide availability. It has been proved to be a powerful tool for studying the quantum manifestations of classical chaos and a typical example for understanding the conception of chaos.In the past years, closed-orbit theory has successfully been applied to H~, H as well as to the highly excited states of the alkali metals atom in various external fields, the validity of the closed-orbit theory has been fully confirmed, and the recurrence spectroscopy, scaling property and energy statistics have also been developed. In recent years, Absorption properties of atoms in Time-dependent oscillation external field (radio frequency field) have been noticed because it is universal that many scientific experiences and applications involve time-dependent fields. However, when takes time-dependent field into account, the initial and final angles, the period of each closed orbit and the classical action will change which make the problem complicated because the effect of radio frequency field. New modulation phenomena are produced then. In addition, when the external laser pulse is not continuous but limited single pulse or double-pulse, closed-orbit theory cannot applied at first hand and must be modified because of the relationship of the calculational precision of photoabsorption spectra and the pulse width.In this submitted work, we take Li system for example and use both semiclassical closed-orbit theory and time-dependent perturbation method to study the photoabsorption spectra of Li in a radio frequency field and laser pulse field and analyze those properties respectively. The main work is as follows:(1) Using perturbation theory based on semiclassical closed-orbit theory, we discuss the photoabsorption spectra of Li Rydberg atom in radio frequency field. We extend closed-orbit theory to the instance of time-dependent external fields and also study corresponding scaled variable spectra ?recurrence spectra .The results show that the amplitude and frequency are both affect recurrence strength greatly, but the rule and pattern of the effects are different. The more large amplitude of the oscillation field is, the more it weakened the recurrence strength. While recurrence strength are not weakened at some special frequencies. These properties will he discussed at length in the third chapter.(2) We calculated the effect of the Gaussian-shaped single pulse laser on the absorption spectra. The results show that the smaller the pulse width, the weaker the oscillation is.(3) We calculated the population of double-pulse population of Li atom and the results prove the important role of the pulse phase difference in the interference of Rydberg wave packets. Double-pulse laser decreases the amplitude of oscillation in photoabsorption spectra much more.In the case of Li Rydberg atom, different from the formers' works, which are approached the atoms only in the time-independent electric and magnetic fields, this thesis study the photoabsorption spectra in time-dependent fields. For the study of Rydberg wave packet dynamics using laser pulses, this thesis is still in the beginning stage. Our work are not only the extension but also the validation and supplement for the original closed-orbit theory. Moreover, our results possess an important insight for experiment research and for theinvestigating of the systems that are more complicated, such as He and 0. systems, etc.This thesis is divided five chapters. The first chapter is summarization, which chiefly introduces the development of semiclassical closed-orbit theory. The second chapter introduces the application of closed-... |
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